The present invention relates to refrigerated display cases having an ambient air defrost system, particularly to open front refrigerated display cases. Both within the specification and the claims of the present application, all references to refrigeration apparatus or refrigeration operations are intended to include cooling both at a temperature below 32.degree. F., such as associated with frozen food display cases, and in excess of 32.degree. F., such as typically associated with dairy food and fresh meat display cases.
A significant contribution to the refrigeration load in the operation of open display cases is created by heat and mass transfer through the air curtain of the display case. Since the heat transfer is dependent upon temperature differentials between adjacent bands of air to reduce this temperature differential. In addition to the heat transfer from the temperature differentials, transfer also occurs of the moisture from the high concentration of the ambient air of the store to the low concentration of the refrigerated air band. Typically, such problems have been at least partially addressed by the utilization of multi-band refrigerated display cases, with a separate set of fans for propelling air through each of the air conduits. In such multi-band refrigerated cases, the innermost air band is refrigerated, the secondary air band, while cooler than ambient air, is not refrigerated. A tertiary band is also typically utilized which propels a curtain of ambient air across the access opening of the display case.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. By this manner of operation, it is possible to avoid any significant frost buildup within the display case.
Typically within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The first approach involves the use of electric resistance heaters which during a defrost cycle supply heat to eliminate the frost buildup on the coils. These heaters, however, add warmer air to the air conduit for circulation within the case. The particular technique is relatively simple body in its construction and operation. However, since the electrical heaters are high voltage heaters that utilize significant electricity during operation, with the rapidly increasing cost of electricity it has become extremely uneconomical to employ such systems. Furthermore, the warm air circulated in the case can raise the temperature of the case too high. Thus, attempts have been made to find other alternatives to such a system.
A second type of system circulates hot compressed gaseous refrigerant through the refrigeration coils during the defrost cycle. During the defrost cycle, a valve control mechanism shuts off the supply of refrigerant to the refrigeration coils and alternatively feeds superheated compressed gaseous refrigerant through the coils. This hot gas serves to melt any frost buildup that has occurred on the refrigeration coils but simultaneously provides heat within the air conduit which can be circulated through the display case, which again is disadvantageous. Due to the requirement that the system be able to selectively switch between the supply of heated gas and refrigerant to the refrigeration coils, a complicated valving structure must be provided.
The third type of system employed for defrosting display cases relies upon ambient air. It is this general category with which the invention of the present application is concerned. One type of system that employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,403,525; 3,850,003 and 3,937,033, all to Beckwith, et al. Each of these systems uses fans separate from the main air circulating fans. These extra fans are turned on during the defrost cycle for pulling ambient air from outside of the display case into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display case. Such ports are normally closed during the refrigeration cyle and are opened during the defrosting cycle. The Beckwith, et al. '003 patent indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
Finally, a third type of ambient air defrosting system is shown in U.S. Pat. No. 4,144,720 to Subera, et al., which is assigned to the same assignee as the present application. In the foregoing patent application, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this system, reversible fans are employed for reversing the direction of flow of air within the conduits and simultaneously drawing in air from outside of the display case.
Another system employing reversible fans for ambient air defrost is shown in U.S. Pat. No. 4,026,121. This patent, however, refers to short-circuiting the air flow between the primary and secondary air bands for the purpose of supplying warmer air to the primary band.
It has been recognized that an ambient air defrost operation can be incorporated into an open top refrigerated display case as disclosed in U.S. Pat. No. 4,120,174 to Johnson. The Johnson patent illustrates an open top case having a single air conduit extending around the case. During the refrigeration cycle, the air flows in a first direction and during the defrost cycle the direction of the air flow is reversed with ambient air being drawn into the conduit. The quantity of air flow during the defrost cycle is greater than during refrigeration. The defrost air, after passing through the conduit, is expelled in a direction up and over the refrigerated case.
During the defrost operation, as the ambient air passes through the air conduit containing the evaporator coils such air is initially cooled by the frost buildup that exists on the coils. In addition, the air flow is significantly restrained since the openings between the coils are often substantially blocked. While in the multiband display cases the ambient air passing through the second air conduit that encircles the case helps in the defrost operation, such a secondary air conduit requires the utilization of additional fans for circulating the air as well as additional materials for purposes of construction. Consequently, both the single band and multiband display cases have certain inherent drawbacks and it, therefore, has been necessary to make a tradeoff in efficiency and costs between the two types of display cases.
Display cases having a full primary air conduit in which the evaporator coils are located and a partial secondary air conduit have been previously known; see for example U.S. Pat. Nos. 3,690,118 to Rainwater and 3,827,254 to MacMaster et al. The partial secondary conduit has been utilized in order to provide a protective air curtain across the access opening for insulating the primary air curtain established by the refrigeration air conduit from the ambient air outside of the display case. Such display cases, however, have typically utilized electric defrost techniques for defrosting the evaporator coils. While a secondary protective screen is provided, there has been very little, if any, known advantages to the utilization of such a display case with respect to the resulting efficiency of operation.